In the industrial scale manufacture of chlorosilanes from silicon, the silicon starting material is most often a raw silicon which contains 85% by weight or more of silicon. Other components of the raw silicon are mainly iron, aluminum, calcium and titanium, which are converted into their chlorides upon reaction with chlorine or hydrogen chloride. In addition to these metal chlorides, the reaction also produces high boiling point compounds such as hexachlorodisiloxane and pentachlorodisiloxane.
Conventionally, these by-products are separated from the chlorosilanes by distillation. Depending upon the distillation conditions, the residue is obtained in the form of a suspension or a solid which requires a separate processing procedure.
The distillation of the chlorosilanes is performed as completely as possible, because any chlorosilanes which remain in the distillation residue can no longer be reacted into useful products and, therefore, represent a loss. In those cases where the total distillation residue is obtained as a suspension, this suspension has a typical composition of about 30 to 40% by weight aluminum chloride, 2 to 3% by weight iron chloride, 2 to 3% by weight titanium tetrachloride, 35 to 45% by weight hexa- and pentachlorodisiloxane, and the rest silicon tetrachloride and about 1% by weight trichlorosilane.
From German Patent 21 61 641 it is known to react the distillation residue with steam accompanied by the formation of hydrogen chloride. A sufficient reaction takes place, however, only if very large excesses of steam are used, so that a very dilute hydrochloric acid is obtained which must be disposed of.
In order to avoid the formation of dilute hydrochloric acid, German Patent 36 42 285 proposes to perform the hydrolysis of the distillation residue in the presence of additional hydrogen chloride, and to recirculate the unreacted water. In accordance with German Offenlegungsschrift 37 42 614, the residual chloride content in the hydrolysis residue can be further reduced by an additional treatment with air.
All of these methods are costly and some of them produce environmentally objectionable liquid and gaseous waste products.
U.S. Pat. No. 4,690,810 discloses a process for the reaction of chlorosilanes with lime slurry, where the chlorosilanes contain up to 20% by weight of finely divided metals and between 0.1 and 5% by weight of the chlorides of aluminum, iron and titanium. The chlorosilane stream is introduced by means of an immersion tube into a lime slurry with a pH of at least 9, until the pH of the suspension drops to between 7 and 8. In the example of this patent the chlorosilane stream contains 14% by weight trichlorosilane, 76% by weight silicon tetrachloride, 0.16% by weight of other metal chlorides and 10% by weight of elemental solid silicon. 1665 kg of this chlorosilane stream were reacted with 5678 liters of a lime slurry which contained 15% by weight calcium oxide (CaO).
Assuming complete reactions pursuant to ##STR1## it follows from this example that calcium oxide in an amount of 97%, based on the above stoichiometric equations was needed to perform the reaction. The reaction of calcium oxide is complete, and a solid is obtained which can be readily filtered off. The clear filtrate must be disposed of.
This reference gives no guidance for the technical procedure with respect to the processing of residues of a chlorosilane distillation, because virtually pure chlorosilanes are reacted in which the aluminum chloride content is negligibly small and no chlorosiloxanes are present. Moreover, the reference provides no guidance with respect to the filtrate disposal. If a typical distillation residue from the distillation of chlorosilanes, which were prepared by a reaction of raw silicon with chlorine or hydrogen chloride, is reacted with lime milk in about stoichiometric amount pursuant to the teachings of the above mentioned U.S. Patent, a barely filterable, slimy residue and a cloudy filtrate are formed.